Strained semiconductor heterostructures are useful for a wide variety of device applications where a narrowed bandgap can improve device performance. Multilayer semiconductor structures comprising heterojunctions are useful in the fabrication of numerous different electronic and optoelectronic devices, one of the most significant electronic devices being bipolar transistors. Strained heterojunction semiconductor devices are also advantageously used as the channel or source/drain regions of PMOS or NMOS metal oxide semiconductor field effect transistors (MOSFET's). For example, in heterojunction MOSFET devices, a strained channel region enhances carrier mobility within the channel. In strained silicon channel regions, the strained silicon layer typically is formed on a substrate layer of greater lattice parameters than that of silicon. A relaxed silicon-germanium layer is a suitable material for such a substrate layer because of its compatibility with silicon processing. As such, a strained silicon layer formed over a relaxed silicon-germanium layer is particularly advantageous.
One known approach to forming a strained silicon channel layer involves growing a silicon layer on an as-grown relaxed or unstrained SiGe layer. The relaxed SiGe layer is formed by first growing a graded Si1-xGex layer on a silicon substrate where x increases from 0% to 30% over a thickness of about 1.5 microns. Next, a 1.0 micron layer of Si0.7Ge0.3 is grown over the graded layer following a thin Si1-x Gex layer where x decreases from 30% to 0% over a thickness of about 0.03 microns. This approach has several disadvantages including a high epitaxial film cost because it takes approximately 6-8 hours to grow the different layers. Additionally, this approach results in a high concentration of dislocations because of the thick expitaxial layers. In addition to reducing the concentration of such dislocations and reducing costs, a thinner Si1-xGex epitaxial layer provides improved heat conduction, reduced junction capacitance and reduced junction leakage.
Accordingly, methods are needed for forming multilayer heterojunction devices having strained silicon layers formed on relaxed layers, and low defect levels. It would be further advantageous to utilize thinner epitaxial layers for such a purpose.